Yes, the parties’ standard-bearers have already been selected and presented to the public. But the conventions give the parties a chance to shape their images and platforms.

In some years, the parties have emerged from the conventions with sharply contrasting tones. For example, when the Democrats were split over the Vietnam War in 1968, the party’s elites picked Vice President Hubert Humphrey at the convention in Chicago, and the antiwar faction went ballistic. Police and protesters battled in the streets, while pro- and antiwar delegates shouted each other down in the convention hall. Meanwhile, the Republican Party met in Miami Beach and had a tranquil coronation of Richard Nixon. The GOP came out looking better — and went on to win in November. And incidentally, the chaos in Chicago led to the reforms that created the modern nominating system.

The 1992 conventions pitted Republican culture warriors Pat Buchanan and Pat Robertson, whose calls to “take back our country” sounded tone-deaf to many voters, against Democrats Bill Clinton and Al Gore, who projected youth, vitality and progress. The country rewarded their social liberalism in November as George H.W. Bush lost his bid for a second term.

And in 2004, each convention sought to portray its candidate as a war hero. The Democrats made John Kerry’s service in Vietnam a key theme, only to see it tarnished by the swift boat ad campaign. George W. Bush, who did not see combat in Vietnam, trumpeted his strong leadership after the attacks of Sept. 11, 2001, and during the still-popular wars in Iraq and Afghanistan. Bush’s manufactured military career dominated Kerry’s actual one.

Considering how negative the 2012 campaign has been, it would not be surprising if both conventions focused mostly on the flaws and shortcomings of their opponents — a classic “lesser of two evils” election.

2. The nominee’s speech is the most important part of the convention.

Many of us might get our fill of the candidates before the party meetings start. However, other speeches can have a lasting effect on the rest of the campaign.

In 1980, Sen. Edward M. Kennedy’s attempt to challenge President Jimmy Carter for the Democratic nomination failed, but Kennedy’s words at the convention proved memorable. His “the dream shall never die” speech, imploring the party to renew its commitment to economic justice, roused convention-goers to their feet. And his endorsement of his onetime rival helped give Carter a bump in public support.

In contrast, Buchanan’s “culture war” speech dragged down the Bush-Quayle ticket in 1992, turning off moderate voters with moralistic rhetoric.

Convention bounces — higher favorability ratings for a candidate after the party’s meeting — are generally thought to occur because many voters are just beginning to pay attention to the candidates, and their first impression is usually a good one, considering that the candidates can control the setting and message at a convention much more easily than at any other time in the campaign. The bounce may be more a reflection of hype than a measure of sustained support.

But sometimes a convention can kick-start a campaign to victory, such as Bill Clinton’s in 1992. His 16-point post-convention jump in the polls, compared with George H.W. Bush’s five-point rise, was the biggestsince surveys began measuring the bounce in 1964.

In other years, the lack of a bounce has hurt a faltering campaign. Democrat George McGovern didn’t get one in 1972, while Kerry’s favorability rating went down after his 2004 convention. Those were bad signs for both candidates, who fell short in their bids for the White House. Given the latest polls, such a bounce might be more important for Mitt Romney than for Obama.

4. The delegates are a bunch of political hacks on a taxpayer-funded junket.

First of all, the delegates travel to the conventions at their own expense. Second, you do not need to be a current or former elected official to attend. The gatherings are certainly dominated by those with political experience, but ordinary voters — with a little ambition, luck and disposable income — have a decent shot at attending.

The delegate process varies among states, but anyone can apply with the local party office. Each state is allotted delegates in proportion to its population and with regard to its partisan voting history. States that are deeper shades of blue, as measured by Democratic votes for president and for governor, have more delegates invited to the Democratic convention, for example. California has been allocated 611 Democratic delegates this year, while Delaware has 32.

5. There are no surprises.

Even though a vice presidential candidate is now more likely to be selected a few weeks ahead of time, the convention is often a coming-out party, setting the tone for the rest of his or her political career. The prototypical example occurred four years ago when Sarah Palin was thrust onto the scene by John McCain. Her folksy personality charmed or rankled, depending on where you fell on the ideological spectrum. The GOP convention was must-see television simply because the country was discovering this fascinating individual.

And in 2004, a young state senator from Illinois thrilled the Democratic crowd with a speech that showcased his life story and his belief in a better America. We all know what happened to that guy.

While Romney’s running mate, Rep. Paul Ryan, is a rising conservative star in Washington, most Americans don’t know much about him. The Republican convention is his chance to change that. However, with Sen. Marco Rubio delivering the speech introducing Romney, many in the party may be surprised to find themselves wishing that their nominee had made a different choice.

Get ready for balloons, funny hats and lots of speeches. In the coming weeks, the Republican and Democratic parties will officially select their presidential nominees at their national conventions in Tampa and Charlotte, respectively. Until the modern primary and caucus system was established in the 1970s, the conventions held a lot more political significance; they were where the parties actually picked the nominees. Yet, despite the fact that we have known the identities of this year’s nominees for months, the conventions still matter. Here are a few things they do — and don’t — reveal.